The present invention pertains to a fluid dilution sampling apparatus for either proportional sampling or fixed dilution sampling of a fluid. The invention is useful with any source, but has particular application to exhaust analysis. While the invention is illustrated for engine exhaust emission analysis and, in particular, to measuring the mass of particulate matter in the engine exhaust, it may also be used for exhaust analysis for boilers, industrial stacks, and the like.
When a gasoline-or diesel-engined vehicle is driven, carbon particles and condensed high-boiling-point hydrocarbons are emitted from the tailpipe, generating particulate matter (PM) after being diluted and cooled in ambient air. In order to measure PM emissions using simulated driving conditions in the laboratory, a dilution tunnel is traditionally used. Previously, in order to measure PM, only a steady-state test cycle was required and the dilution systems were mainly total dilution types, dubbed full-flow dilution tunnels. These tunnels were very large and could occupy most of the test cell space.
Recently, a new transient engine test has been under consideration as a more realistic simulation of PM measurement than the steady-state test cycle. At the same time, the technique known as partial exhaust dilution sampling has been considered as the basis of measuring particulates from this new transient engine test. Partial exhaust dilution systems work by sampling part of the engine exhaust gas-flow, keeping a constant split ratio (the ratio of exhaust total flow to sampled flow). This is carried out by mixing the sample gas with dilution air inside a small dilution chamber and then the diluted exhaust gas passes through filters where the particulate material is deposited.
For proportional sampling, the control of the dilution air requires fast response to control inputs. There are two major factors that affect delay in flow control during a transient cycle: firstly, the time delay of the exhaust-gas flow rate measurement itself and the delay for the sample to reach the sampling point from the engine (exhaust measuring point); the second factor is the delay in the flow control of the dilution tunnel. The first factor can be corrected by using a predictive control method. Overcoming the problems posed by the second factor, i.e., speeding up the response time of the dilution tunnel flow control, has been addressed by the following:                A first approach replaced the traditional vortex blower rotation control method (used on full flow dilution systems) with a flow control method that used a piezo-valve to control the flow rate of compressed air used for dilution. This approach came from a design for a hot-wire type mass-flow controller that uses a piezo control valve. Such devices are in common use, although their accuracy and response speed are not satisfactory for use in this application.        A second approach combines a piezo-valve with a venturi flowmeter. In addition to these components, by using a critical flow orifice (CFO) with the piezo-valve, the response time can be reduced to 0.2 second in open-loop control of the piezo-valve. This technique is still too slow.        